This and a companion project (Z01 HD001123-11) investigate auditory communication in primates. The overall goal of these studies is to provide a comprehensive understanding of primate auditory communication in terms of development, neural mechanisms, endocrine factors, and social context. Two species, the squirrel monkey and the common marmoset, are the main subjects of study, with additional data collected from other species where appropriate. The present project studies primate communication from a bioacoustic and ethological perspective, focusing on the detailed acoustic structure of vocalizations produced in natural settings, and the relationship of structure differences to differences in age, gender and experience, as well as the broader factors of social context and genetic background. New findings focused on the perceptual side of auditory communication, using playback techniques to examine perceptual preferences as measured by vocally responding to the stimulus. A close-range affiliative vocalization given mainly by adult female squirrel monkeys, the chuck, was tested for responsiveness based on the familiarity of the sound (due to the acoustic details of the chucks of individual vocalizers) to the listener. Two experimental groups were used. Both had juvenile as well as adult females. Juvenile females (under 2 years of age) never vocally responded to a playback stimulus. Adult females responded differentially to the chucks of familiar group members, confirming a study conducted last year. Digital manipulation of the familiar chuck stimuli (by selectively removing parts of the chuck or changing overall frequency range) showed that responders prefer the intact chuck. This suggests that the acoustic structure of the entire chuck is the perceptual unit used by squirrel monkey females in their natural conversational communication.